Research conducted over the past decade has demonstrated the value of isokinetic exercise from the standpoint of rehabilitating injured human joints and associated muscle groups as well as training joints and muscle groups for improvement of human performance. The term "isokinetic" refers to the exercise concept that involves restricting the movement of a portion of the body about a particular anatomical axis of rotation to a constant rotational velocity. This is achieved by applying an accommodating resistance to the contracting muscle, that is a constantly varying resistive force. This resistive force changes in value throughout the range of motion of the limb in a manner which mimics the varying amount of force that the associated muscle group is able to generate at various points throughout the contraction.
The observation that the amount of force which a muscle group generates varies throughout the range of motion of the associated joint may be explained in terms of anatomical axis of rotation (i.e., a variable biological lever length advantage), enzymatic profile (i.e., intracellular contractile and metabolic protein composition), musculo-tendinous length tension relation and ballistic considerations. An example of this phenomenon can be shown in the knee joint extension during which the quadriceps muscle is seen to develop peak torque at about the midrange of rotation.
Conventional methods of "free weights" exercise require the muscle to act against a load which cannot be greater than the least torque developed at the weakest point in the range of motion of the joint. Thus, with free weights the muscle operates at a reasonable work load in only a small portion of the overall range of motion and does not experience optimal loading during the stronger points in the range of motion.
Semi-accommodating resistance exercise such as is provided in some cam-based exercise systems, wherein the load on the muscle is biased and semi-variable, are at best approximations to the variations in force generated by the particular muscle groups sampled from a cross-section of individuals. This approximation of variable force generation, which may be visualized as a quasi-bell shaped curve of force plotted against degrees of range of motion, is used to shape a cam to control application of the resistive force in a semi-accommodating, semi-variable manner.
Isokinetic exercise systems, on the other hand, provide completely accommodating resistance which offers a variable force opposing muscle contraction in a manner which imitates the variable force generated by the involved muscle group. In this type of system, the rotational velocity of the lever arm to which the human limb is attached is constrained to a maximum permitted value and any force exerted by the limb which tends to accelerate the lever arm beyond that maximum value is matched with an accommodating resistance. Accordingly, the muscle group involved may operate at its optimal tension development throughout the entire range of motion. The net rehabilitation benefit or the net gain in human performance in this training modality is substantially greater than that achieved with conventional exercise modes.
The greater benefits of isokinetic exercise can also be explained in terms of the effect on the recruitment pattern of specific muscle fiber types. It has been established that skeletal muscle is an admixture of at least two distinct cell types. One type of cell is relatively large and rich in anaerobic enzymes which have the capacity to carry on the cell's metabolic needs without oxygen. The other cells are smaller and rich in aerobic components which rely on the presence of oxygen to supply the cell's metabolic requirements. Research has shown that, at large loads and low speeds of contraction, the muscle cell type primarily involved in the exercise is the anaerobic, large diameter cell. Conversely, at low loads and high speed of contraction, the aerobic, smaller diameter cells are primarily involved.
A rehabilitation program which imposes a work load on the muscle groups at either of the extremes of load and velocity will be neutrally recruiting one particular muscle cell type. Therefore, it is essential that the muscle be exercised at the center of the force-velocity curve for maximum rehabilitation benefit. This can best be achieved with isokinetic exercise and specific speed selection for the constant contractile velocity of the involved muscle group.
Isokinetic exercise systems of the prior art (for example, the system shown in Perrine U.S. Pat. No. 3,465, 592) involve a fixed length lever arm which is attempted to be adjusted accurately to the length of the limb being exercised by aligning as closely as possible the anatomical axis of rotation with the fixed machine axis of rotation. However, this alignment can only be approximated prior to the onset of exercise and hence the rotational velocity of the limb will differ from the rotational velocity of the lever arm. Moreover, a further complicating factor is that the axis of rotation of a human joint is dynamic and shifts during the exercise motion. This is true of the knee joint and is especially pronounced in the shoulder joint. Even if the position of the involved joint is mechanically constrained, the anatomical axis of rotation will shift during the exercise motion. As a result, the constraining of the fixed length lever arm to a maximum permitted angular velocity means that the angular velocity of the involved limb about the anatomical axis of rotation will vary.
An additional disadvantage of the fixed length lever arm systems of the prior art is that substantial joint compression is introduced by constraining the human limb to a fixed path of rotation at the point of attachment to the lever arm. This joint compression produces an uncomfortable level of pain in certain individuals with joint problems. More seriously, the magnitude of joint compression produced in prior art systems precludes early initiation of rehabilitative exercise in patients which have undergone surgery on the involved joint.
A further disadvantage of the fixed length lever arm systems of the prior art is that the ability to isolate muscle groups involved in extension and flexion is substantially reduced and it is observed that substantial motion of other portions of the body are involved during the exercise motion. This motion of other portions of the body brings into play work activity by the associated muscle group and makes it impossible to isolate the work performed by the muscle groups directly involved in the exercise motion of interest or makes it such that the different muscle groups producing flexion and extension are not actually isolated during respective portions of the exercise motion. The unnatural feeling of the exercise motion is a deterrent to patient interest and willingness to follow the exercise regimen over the period of rehabilitation.